1. Field of the Invention
The present invention relates to highways of a PBX (private branch exchange), and more particularly, to a technique for distributing highways to subscriber cards of a PBX.
2. Description of the Related Art
In general, a PBX is contained within a cabinet. The cabinet has a plurality of shelves therein. Cards for performing various switching functions demanded from the exchange are inserted into respective slots of these shelves. For instance, a card including a circuit for switching a subscriber signal is installed on a control shelf of the exchange, and a card including a circuit for accommodating subscribers is installed on a line shelf. A highway cable is connected between the control shelf and the line shelf.
Typically, one highway is comprised of 32 channels each having 8 bits and synchronized with a frame synchronization of 8 KHz. Therefore, a PCM (Pulse Code Modulation) data speed of one highway is 2.048 MHZ (=8 KHZ*8 bits*32 channels). A time switch, which is a PCM switch, uses 8 highways generally, and therefore, it has 256 channels (=32 channels*8).
In an earlier highway distribution apparatus in a private branch exchange, a time switch has 8 highways HW1, . . . , HW8. 16 line cards each have 16 channels and interfacing with the exterior. One highway is connected to two line cards each having 16 channels.
According to the number of channels, the line card is classified into a 32-SLI (Single Line Interface Card) using 32 channels, a 16-SLI using 16 channels and an 8-SLI using 8 channels. One highway may be fixedly connected to 2 16-SLIs. However, if the 8-SLI or 32-SLI is actually used, operation may be impossible or the highway channel may bring about a loss. In other words, assuming that the time switch is set to 16 channels per line card, operation is impossible in an ISDN (Integrated Services Digital Network) or an ATM (Asynchronous Transfer Mode) because it needs 64 channels, and there is loss of channels in the 8-SLI. Moreover, if the line card requires 32 channels, only one of two line cards to which one highway is supplied is used, and the other line card can not be used.
The following patents each discloses features in common with the present invention but do not teach or suggest the specific technique for distributing highways in an exchange in accordance with the present invention: U.S. Pat. No. 4,796,294 to Nakagawara, entitled Electioonic Telephone Exchange And Method Of Connecting Ports Of The Exchange, U.S. Pat. No. 4,736,409 to Hasegawa et al, entitled Control Data Transmission Systems For Private Branch Exchange, U.S. Pat. No. 4,674,083 to Rachkin, entitled Time Division Multiplexed Switching Structure For PBX, U.S. Pat. No. 4,140,877 to Joslow et al., entitled Multiple Highway Time Division Multiplexed PABX Communication System, U.S. Pat. No. 5,161,155 to Dyer et al., entitled Automatic Telecommunications Systems, U.S. Pat. No. 4,694,452 to Beckinger et al., entitled Switching Configuration For A Telecommunications System In particular A PBX System With Subscriber Lines, Trunk Groups, And Interface Modules, U.S. Pat. No. 5,151,896 to Bowman et al., entitled Modular Digital Telephlone System With Fully Distributed Local Switching And Control, U.S. Pat. No. 5,682,385 to Garcia et al., entitled Enhancement For A Multiplexing Telecommunications Interface, and U.S. Pat. No. 5,581,553 to Cave et al., entitled Distributed Switching Architecture.